How to make a backup of my 128GB iPhone directly on external drive?

hey guys,

I was trying to answer this basic question: How to make a backup of my 128GB iPhone directly on external drive?

After searching Google I found different threads and managed to get the answer I was looking for, basically creating a symbolic link to a USB Drive I always have connected to my MacBook Air (256GB storage when purchased)

My OSX source path for such backup is:

/Users/marcofama/Library/Application Support/MobileSync/Backup

The destination path (which exists and works well) is

/Volumes/marcofama/iPhoneBackup

So I tried launching the below command:

sudo ln -s /Volumes/marcofama/iPhoneBackup "/Users/marcofama/Library/Application Support/MobileSync/Backup"

The link is created, I can see the "arrow icon" in the finder.

Problem is: when I try backing up the iPhone, I see folders being created on my USB drive (like 1a, 0b, cc, stuff like that) but at some point still Mac OSX complains:

Could not back up the iPhone “iPhone di Marco Famà” because not enough free space is available on this Mac.

Removing files and emptying the Bin will free up additional space.

Do you know why and how I can fix this? (Please do not suggest to buy a 512GB new Mac :) ha ha)

Thanks.